The present invention relates generally to alternating current motor drives and, more particularly, to a variable speed induction motor drive of the type including semiconductor devices capable of being turned on and turned off in response to control signals.
There are a number of instances where it is desired to have a variable speed motor drive using an alternating current induction motor. A number of inverter drives for providing this function are known and it is also known that the standard, so called, square wave output of such inverter drives tends to provide harmonics of the fundamental square wave frequency to the induction motor. These harmonics can cause the motor to overheat and/or to develop torque pulsations in its mechanical output. This is particularly true in the retrofit market where it is often desired to provide an inverter drive having variable speed capabilities to an existing or fixed speed motor installation without requiring motor derating as a result of the inverter output waveshape. It is known in such drives to place a large load filter capacitor at the output of the inverter (the motor input) in order that the input current and voltage to the motor is a sine wave. However, the 120 degree rectangular current waveshape produced by such inverter drives normally produces harmonics of "n" times the fundamental frequency wherein "n" is an odd number, not a multiple of the number of phases. The electrical reasonance of the load filter capacitor with the motor leakage inductance can be excited by different fundamental frequency harmonics over the motor speed range. Continuous operation of the drive at speeds where that reasonance is being excited can cause torque harmonics which make motor performance unacceptable or potentially damaging to the load on the motor.
A number of techniques have been employed to eliminate selected harmonics in voltage source inverters. See, for example, that described by Turnbull in 1963 IEEE transactions, paper 63-1011, entitled "Selected Harmonic Reduction in Static AC/DC Inverters". Hasmuth Patel generalized this approach in his doctoral dissertation at the University of Missouri in 1971. Several Japanese authors have discussed the pulse width modulation of inverter drives for current source inverters to improve current waveshape and some also include a load capacitor of a relatively small magnitude on the inverter output, primarily for the purpose of high frequency filtering. These drives have not, however, utilized a large load capacitor as a primary means of improving the motor current waveshape and maximizing the power factor of the drive. Neither have these references employed the selective providing of notches within the fundamental waveshape to control the harmonics applied to the capacitor/motor combination as a function of the fundamental frequency and the size of the electrical components.